When printing colored documents, it is common to print black objects on a colored background. Commonly the objects are text, although the objects also may be graphic items as well. When printing black on color, it is usual to remove the color data that coincides with the black objects. In other words, a hole is created in the colored part of the image that matches the shape and size of the black object to be printed on top of the color.
In conventional printing, a defect may arise if the registration between the color part of the image and the black shifts. Since the black and color parts are not usually printed simultaneously, mechanical shifts in the printing media between the printing of the black and the color parts of the documents may give rise to such a shift.
In such a case, the black objects may have a white area adjacent thereto due to the shift of the image and possibly a darker edge as well. Such an unintended white border or darker edges are known as registration defects. The white may arise because the hole that was cut to match the black object is not exactly covered by the black object due to the unintended shift between the black and color parts. The dark edge may arise because the black is printed on top of color making the resulting image darker where the objects overlap. A variety of conventional techniques have been used to reduce or eliminate registration defects.
Many of the conventional techniques used to eliminate or reduce registration defects require extensive processing of the document to be printed as it is converted from a page description format to page images. The processing may require heavy use of the computer resources used to convert the document to page image form, thus slowing down the speed of any attached printer.
A registration defect problem may arise if the document is received by the printer already in page image form. In such a case, it is not easy to identify the black objects and the matching space in the color part of the document, since the page images are usually in raster form.
Thus, it would be desirable to find a method of correcting for registration defects that does not require extensive processing and operates on a rasterized image as opposed to the page description language stage of the document.